Purification of tungstates



United States Patent PURIFICATION OF TUNGSTATES Pai Yen Loung, Glen Cove, N.Y., assignor to Wah Chang gorfioration, New York, N.Y., a corporation of New No Drawing. Application November 18, 1957 Serial No. 696,929

4 Claims. (Cl. 23-51) This invention relates to the purification of tungsten values and specifically to the removal of phosphorus and arsenic impurities from tungsten values. The application is a continuation-in-part of my copending application, Serial No. 466,274, filed November 2, 1954, now abandoned, and similarly entitled.

' Theprincipal object of this invention is to provide a simple, effective and economical process for the separation of phosphorus and arsenic impurities from tungsten values. In accordance with this'invention the tungsten values, if they are not available as an alkali metal tungstate, are first placed in the form of alkali metal tungstate solution which is then purified to effect the removal of phosphorus and arsenic impurities by heating and hot filtering the solution at an adjusted alkaline pH. The removal of the phosphorus and arsenic impurities is achieved by crystallization of the alkali metal tungstate from such a solutioncontaining a suitable amount'of alkaline metalhydroxide, after heating and upon hot filtering the tungstate crystals. In accordance with this invention the alkali metal tungstate solution should have an initial alkali metal hydroxide concentration in the range of about 0.4% to about 4% by weight of-the solution. The pH of the solution should be so adjusted to be preferably around 13 and, more generally, in the range of 12 to 14.

An alkali metal tungstate solution having been prepared in accordance with the above principles, is then evaporated to reduce its original volume to a point where precipitation of alkali metal tungstate crystals takes place.

contain those impurities in amount which is substa n tially reduced. v

As the solution is evaporated and its aqueous component thereby diminished the relative amount of alkali metal hydroxide will increase. If as a result the pH reaches 14 or above there will exert a tendency to precipitate phosphorus impurities, therefore in the preferred practice of the invention when utmost freedom from im--- purities is desired and the removal of relatively large amounts of water by evaporation is contemplated the pH should be initially adjusted to a lower part of the range.

In the preferred practice of the invention the mother liquor resulting from the first filtration is treated by acidification to adjust its pH within the range of 12 to 14 and evaporation and subsequent filtering is carried out to obtain a second crop of alkali metal tungstate crystals.

if desired, this step may again be repeated and repeated.

As the successive crops of alkali metal tungstate crystals are obtained the amount of phosphorus and arsenic present therein will increase slightly. There is an end point beyond which it is not practical to repeat the step of adjusting the pH of the mother liquor and again evaporating to induce precipitation'and filtering to remove the precipitated crystals of alkali metal tungstate depending upon the reduction of impurities desired and the'cost of Example I 3v liters of sodium tungstate solution, with pH of about 14, obtained from acid digestion of Korean scheelite which had not been previously acid leached to remove phosphorus andwhichhad been treated with NaOH, was

first evaporated to about one-half of its original volume, filtered hot, and first crop of sodium tungstate crystals obtained. Acid was then added to keep the pH at 14, and the operation repeated to obtain the second, third and fourth crops of sodium tungstate crystals. The last filtrate was cooled to room temperature with water, and a mixture of crystals containing tungsten and phosphorus The exact amount of volume reduction will, of course, obtained. The following table represents the results:

Distribution W01, P, percent percent Sodium tungstate solution (Head calculated) 3,000 00.... 100 100 1st NazWOr crystals 336 gms... 24. 56 2. 04 2d NmWOr erystals 675 gms. 48. 25 18.37 3d N a1W04 crystals- 263 gms. 18. 72 7. 48 4th NaWO4 crystals 95 gms 5. 35 5. 44 Mixed W-P crystals 28 gms. 1. 28 57. 83 Mother liquor 150 cc... 1.84 8. 84

depend upon the original concentration of the alkali metal tungstate in the solution. As a general rule, and from a commercial standpoint, it is economical to form the original alkali metal tungstate solution in a concentration of about 200 grams per liter by weight of alkali metal tungstate. When the concentration is thus adjusted a reduction in the volume by heating the solution and evaporating it of about 400 to 500 grams per liter will often be sufiicient to induce the desired precipitation of alkali metal tungstate crystals. 7

The mixture of hot solution and alkali metal tlmgstate crystals is then filtered to remove the tungstate crystals and it will be found that the crystals so removed are either substantially free of phosphorus and arsenic impurities or Example 11 3 liters of sodium tungstate solution with pH of about 14 obtained from acid digests of Korean scheelite which Patented July 7,1959

3 hadnot'been previously acid leached to remove phos phorus and which had been treated 'with NaOH was first evaporated to two-thirds, then one-fifth of the original volume, cooled at 5 C., evaporated to about one-tenth of the original vo1un1e,"filtered after each step.

Having thus described my invention I claim:

1. The method of removing arsenic and phosphorus impurities from alkali metal tungstates containing such impurities in which the alkali metal tungstate is dissolved in water together with an amount of alkali metal hydrox- 5 The following table presents the results: 1de Wl'llCl'l hydroxide, in amount, constitutes about 0.4 to

Assay Distribution W0. P Wot, P,

percent percent Sodium tungstate solution (Head calculated) 3,000 can" 365.32 g.ll 100 1st NaiWO; crystals 600 gms.'. 77.22% 42.28 17.65 2d Naiwol'fllysllflls 693 gms... 75.38% 47.66 16.47 3d NmWOl crystals..- 76 gms 69.32% 4. 81 2. 35 4th Nallvot crystals 27 gms 52.07% 1.28 58.82 5th-Na7W0| crystals 61.83% 1. 97 Mother Liquor 7121.56 g./l 2.00 4

. With the fourth group of sodium tungstate crystals produced by this example eliminated, the total sodium tungstate crystal production was '1404 grams with 0.044% of phosphorus to give a 96.72% recovery of 'trioxide from the original sodium tungstate solution and a 63.53% elimination of phosphorus. As described above in connection with the-flow sheet, the mother liquor may be recycled.

The above examples deal with the common alkali metal tungstate solution, -i.e., the sodium species, and generally,.

in'commercial operations sodium tungstate solutions will be formed because of their convenience and cost. The invention is operable, however, when the other alkali metal tungstates are used.

It will be noted that among the advantages of the process of this invention .is asubstantial recovery of'tung sten values relatively free from phosphorus without the use of expensive chemical agents. The process of this invention has the further advantage that the operations are conducted under the alkaline conditions which permit the use of relatively inexpensive processing kettles or containers. In addition the by product salts are unobjectionable substances which are readily disposed of.

about 4 percent by weight of the final solution, adjusting the pH of the solution to a value of '12 to 14 and, thereafter, heating said solution to cause evaporation of its aqueous content to a point where substantial amounts of alkali metal tungstate crystals are precipitated therefrom and filtering the heated solution to remove said crystals.

2. The process of claim 1 in which the alkali metal component of the named compounds is sodium.

3. The process of claim 1 in which the mother liquor separated from said crystals from said filtration is adjustedto a pH of 12to 14 and the liquor is again heated to evaporate further amounts of the aqueous component thereof and induce the precipitation of further amounts of alkali metal tungstate which are then filtered from the hot solution.

4. The process of claim 3 in which the alkali metal component of the named compounds is sodium.

Carosella June 12, I951 

1. THE METHOD OF REMOVING ARSENIC AND PHOSPHORUS IMPURITIES FORM ALKALI METAL TUNGSTATES CONTAINING SUCH IMPURITIES IN WHICH THE ALKALI METAL TUNGSTATE IS DISSOLVED IN WATER TOGETHER WITH AN AMOUNT O F ALKALI METAL HYDROXIDE WHICH HYDROXIDE, IN AMOUNT, CONSTITUTES ABOUT 0.4 TO ABOUT 4 PERCENT BY WEIGHT OF THE FINAL SOLUTION, ADJUSTING THE PH OF THE SOLUTION TO CAUSE EVAPORATION OF ITS AFTER, HEATING SAID SOLUITON TO CAUSE EVAPORATION OF ITS AQUEOUS CONTENT TO A POINT WHERE SUBSTANTIAL AMOUNTS OF ALKALI METAL TUNGSTATE CRYSTALS ARE PRECIPITATED THEREFROM AND FILTERING THE HEATED SOLUTION TO REMOVE SAID CRYSTALS. 